Zip lists in Python

zip takes a bunch of lists likes

a: a1 a2 a3 a4 a5 a6 a7...
b: b1 b2 b3 b4 b5 b6 b7...
c: c1 c2 c3 c4 c5 c6 c7...

and "zips" them into one list whose entries are 3-tuples (ai, bi, ci). Imagine drawing a zipper horizontally from left to right.

In Python 2.7 this might have worked fine:

>>> a = b = c = range(20)
>>> zip(a, b, c)

But in Python 3.4 it should be (otherwise, the result will be something like <zip object at 0x00000256124E7DC8>):

>>> a = b = c = range(20)
>>> list(zip(a, b, c))

When you zip() together three lists containing 20 elements each, the result has twenty elements. Each element is a three-tuple.

See for yourself:

In [1]: a = b = c = range(20)

In [2]: zip(a, b, c)
Out[2]: 
[(0, 0, 0),
 (1, 1, 1),
 ...
 (17, 17, 17),
 (18, 18, 18),
 (19, 19, 19)]

To find out how many elements each tuple contains, you could examine the length of the first element:

In [3]: result = zip(a, b, c)

In [4]: len(result[0])
Out[4]: 3

Of course, this won't work if the lists were empty to start with.